### API Change

- Avoid creating new controller revisions for statefulsets when cache is stale ([#67039](https://github.com/kubernetes/kubernetes/pull/67039), [@mortent](https://github.com/mortent))
- Fixes issue when updating a DaemonSet causes a hash collision. ([#66476](https://github.com/kubernetes/kubernetes/pull/66476), [@mortent](https://github.com/mortent))

- Fixes a regression in v1beta1 PodDisruptionBudget handling of `strategic merge patch`-type API requests for the `selector` field. Prior to 1.21, these requests would merge `matchLabels` content and replace `matchExpressions` content. In 1.21, patch requests touching the `selector` field started replacing the entire selector. This is consistent with server-side apply and the v1 PodDisruptionBudget behavior, but should not have been changed for v1beta1. ([#...

Certificate management has become more robust in 1.15, with kubeadm now seamlessly rotating all your certificates (on upgrades) before they expire. Check the [kubeadm documentation](https://github.com/kubernetes/website/blob/dev-1.15/content/en/docs/reference/setup-tools/kubeadm/kubeadm-alpha.md) for information on how to manage your certificates.

The kubeadm configuration file API is moving from v1beta1 to v1beta2 in 1.15.

## Introduction to 1.9.0

Kubernetes version 1.9 includes new features and enhancements, as well as fixes to identified issues. The release notes contain a brief overview of the important changes introduced in this release. The content is organized by Special Interest Group ([SIG](https://github.com/kubernetes/community/blob/master/sig-list.md)).

t".Request).{Do,DoRaw,Stream,Watch}` now require callers to pass a `context.Context` as an argument. The context is used for timeout and cancellation signaling and to pass supplementary information to round trippers in the wrapped transport chain. If you don't need any of this functionality, it is sufficient to pass a context created with `context.Background()` to these functions. The `(*"k8s.io/client-go/rest".Request).Context` method is removed now that all methods that execute a request accept...

- [`KT-53846`](https://youtrack.jetbrains.com/issue/KT-53846) K2 / Context receivers: ClassCastException on secondary constructor of class with context receiver
- [`KT-58621`](https://youtrack.jetbrains.com/issue/KT-58621) K2: Private class shadows public function defined in the same package

* When performing a GET then PUT, the kube-apiserver must write the canonical representation of the object to etcd if the current value does not match. That allows external agents to migrate content in etcd from one API version to another, across different storage types, or across varying encryption levels. This fixes a bug introduced in 1.5 where we unintentionally stopped writing the newest data. ([#48394](https://github.com/kubernetes/kub...

### API Change

- Client-ca bundles for the all generic-apiserver based servers will dynamically reload from disk on content changes ([#83579](https://github.com/kubernetes/kubernetes/pull/83579), [@deads2k](https://github.com/deads2k))